The present invention relates generally to a digital circuit for transmitting information with a binary, multilevel, signal defined by first and second values different from each other or for transmitting information with a multilevel signal defined by a plurality of predetermined values different from each other, and more particularly to a digital circuit for shaping waveform of an input and an output signal to reduce undesirable radiations.
In a digital circuit, a signal processing and an information transmission are carried out by means, for example, of a binary signal, i.e., a first value (or a logical xe2x80x9c1xe2x80x9d) and a second value (or a logical xe2x80x9c0xe2x80x9d). Taken as a common example of such a digital circuit is a TTL-IC (hereinafter referred to as xe2x80x9cdigital ICxe2x80x9d). In general, a noise margin is set to such a binary signal processed in a digital IC so that the binary signal can be discriminated at the signal-receiving side.
A basic waveform of a binary signal processed in a common digital IC is shown in FIG. 10. As shown in FIG. 10-(a), when a value of a binary signal varies within a range between 0 and +5 volts, it is determined at the side receiving the binary signal that the signal is an input of a first value (a first presumption value) if the value of the input signal falls within a first presumption range. Similarly, it is determined that the signal is an input of the second value (a second presumption value) if the input signal falls within a second presumption range.
The first presumption range is, for example, from +2.0 volts to +5 volts and the second presumption range is from 0 volt to +0.8 volt at the input side.
At the side for outputting the binary signal, taking into consideration the noise margin, a value within a range from a minimum output value (+2.4 volts) to a maximum output value for logical xe2x80x9c1xe2x80x9d is outputted as an output signal in the case of the first value, and a value within a range from a maximum output value (+0.4 volt) for logical xe2x80x9c0xe2x80x9d to the ground level is outputted as an output signal in the case of the second value.
When an output signal is produced based on a given input signal (for example, an operation result), if the value of this input signal lies around a threshold value which does not belong to logical xe2x80x9c1xe2x80x9d (or a high level) nor logical xe2x80x9c0xe2x80x9d (or a low level), e.g., if the value is within the range from 1.2 volt to 1.4 volt in the case of TTL, high-level output means for producing a high-level output signal and low-level output means for producing a low-level output signal may simultaneously be activated. If these means are activated simultaneously, a current from the power supply, i.e., a spike current of the power supply increases, as a result of which the operation of the circuit becomes unstable. This situation can substantially be avoided by shortening the time period when the value of the input signal resides in the vicinity of the above-described threshold value. That is to say, it is desirable that the waveform of a binary signal processed in a digital IC have leading edges rising and trailing edges falling substantially vertically. If the leading and trailing edges are gentle a jitter will be caused.
In the meantime, as the waveform at the leading and trailing edges are steeper or approach the right angle, its higher harmonic components increase. Such harmonic components will cause undesirable radiations. If a signal containing such harmonic components is outputted as it is, undesirable radiations (radio waves) are emitted from a transmission path of the output signal into the space. These radiations may have adverse effects on neighbouring electronic devices. For this reason, in the conventional digital ICs when a signal is outputted its waveform is shaped to decrease the harmonic components (undesirable radiations).
The shaping performed to reduce harmonic components is, in reality, a processing to eliminate the abrupt change in waveform. Such a processing is effected on the entire waveform of the output signal. Specifically, the output signal is brought into waveform which exhibits more gentle changes, in other words, into dull waveform as a whole.
One example of such shaping of waveform is shown in FIG. 10-(b). As shown in the figure, when the waveform shaping to eliminate harmonic components has been performed, even the change between the high level and the low level where an abrupt change is desired follows a gentle curve connecting the two levels, as a result of which the time period when the waveform exhibits an abrupt change is limited to a very short period. The waveform exhibiting such a change will cause a problem that timing (an instant of change in value) of the relevant digital signal is not transmitted accurately.
Although the measures of the problem of undesirable radiations have been described above for a digital circuit in which a binary, multilevel, signal is the transmission signal, it is also known that similar problems occur in a multilevel digital circuit in which a signal processing is carried out by processing three values such as xe2x80x9c1, 0, xe2x88x921xe2x80x9d or more values.
Therefore, the present invention has been made in view of the above problems and has its object to provide a digital circuit which can accurately transmit timing of change in value of a digital signal or an instant of change in signal value and can reduce higher harmonic components (undesirable radiation components) or a magnitude of high frequency components (a magnitude of undesirable radiations).
A digital circuit according to the present invention comprises a waveform shaping section to which first and second presumption values for defining first and second presumption ranges in which it is deemed as a value of the binary, multilevel, values or more than two presumption values for defining more than two presumption ranges in which it is deemed as a value of the multilevel values are set, the waveform shaping section performing a waveform shaping for eliminating higher harmonic components of a waveform or reducing a magnitude of the higher harmonic components in each of the presumption ranges, and maintaining abrupt changes of the waveform in a transition period from one of the presumption ranges to another.
With this digital circuit, a waveform from which harmonic components have been eliminated or in which a magnitude of harmonic components has been reduced is generated in the presumption ranges for discriminating the first and second values or for discriminating the multilevel values, whereas a steep state of the waveform is maintained in other ranges during the time period when a transition is made from one presumption range for the presumption of one predetermined value to another presumption range for the presumption of another desired predetermined value.
As a result, changing instants of a digital signal are accurately transferred in the transmission and reception of the signal, and the undesirable radiations can be reduced.
In another digital circuit according to the present invention, the waveform shaping section comprises a comparing section for comparing a value of an input signal with presumption values defining the first and second presumption ranges or with presumption values defining the presumption ranges for the multilevel values, and a time constant section for changing a time constant defining a changing rate of the waveform of the input signal when it is detected based on a comparison result of the comparing section that the value of the input signal is within any one of the presumption ranges.
A further digital circuit according to the present invention comprises an electromagnetic interference (EMI) elimination filter for passing the input signal therethrough and a switch for causing the input signal to be supplied to the electromagnetic interference elimination filter in accordance with a comparison result of the comparing section.
In a further digital circuit according to the present invention, the waveform shaping section shapes the waveform by means of a zener diode and a resistor which are serially connected for passing the input signal therethrough.
In a further digital circuit according to the present invention, the waveform shaping section comprises two zener diodes and a resistor for passing the input signal therethrough, and shapes the waveform by means of the two zener diodes arranged in a reversed relation to each other.
In a further digital circuit according to the present invention, the comparing section outputs such a comparison result that values of the input signal are integrated into the predetermined binary values or multilevel values.
In a further digital circuit according to the present invention, the time constant section sets a waveform changing rate of an output signal to a value in the range of 5 to 50% of a pulse width when a value of the input signal exceeds a predetermined reference value which defines the presumption range.
In a further digital circuit according to the present invention, the waveform shaping section is arranged in an input stage. In a further digital circuit according to the present invention, the waveform shaping section is arranged in an output stage. In a further digital circuit according to the present invention, the waveform shaping section is arranged in input and output stages.
In a further digital circuit according to the present invention, the waveform shaping section is constructed so as to be mountable on a circuit for transmitting information by means of the binary or the multilevel signal.
As a result of the above constructions, changing instants of a digital signal is accurately transferred in the transmission and reception of the signal and the undesirable radiations can be reduced.